This invention relates to electrically conductive polyimide articles and to a process for making such articles. More specifically, the invention relates to polyimide articles of intermediate electrical conductivity which can be made by a manufacturing process utilizing a specific solvent extraction step.
Polyimide articles, such as films, are usually manufactured in a two-stage process. The first stage is a polycondensation reaction between a dianhydride and diamine in a suitable solvent. This polyamic acid solution is then fabricated into a shaped article, e.g., a film, by solvent casting and removal of the solvent. In the second stage, the resulting polyamic acid film is either converted chemically or thermally to the polyimide by the removal of two molecules to water. Solvent extraction from shaped articles of aromatic polyamide as a step in the conversion process has been disclosed in several references, including Japanese Patent Application No. 51-122167; U.S. Pat. Nos. 3,961,009; 4,346,215 and 4,470,944.
Canadian Pat. No. 708,896 discloses polyimide film materials manufactured to exhibit electrical conductivity by virtue of the addition to the film material of finely-divided electricallyconductive particles. The patent discloses conversion of polyamide acid to polyimide by a chemical process, involving batchwise soaking of polyamide acid film, on glass plates, in a bath of acetic anhydride and pyridine. The resulting polyimide films have relatively low electrical conductivity. Thermal conversion of polyamide to polyimide, on the other hand, results in a film material of relatively high electrical conductivity. The Canadian patent does not suggest means for controlling the degree of electrical conductivity except as such control might be expected by use of different concentrations of conductive particles.
It has now been found that polyimide articles having intermediate conductivities can be prepared by a manufacturing process using a solvent extraction step in the later stages. By intermediate conductivity it is meant that the conductivity of the polyimide article is intermediate to that of a corresponding article manufactured using a chemical conversion process as and that of a corresponding article manufactured using a thermal conversion process, as more fully described below. None of the known prior art suggests that extraction steps should or could be used in manufacturing electrically conductive polyimide articles. Polyimide articles of this invention having intermediate electrical conductivity may be suitable for use in electronics, copier belts, heating tape, electromagnetic interference shielding, and related applications.